This application relates to and incorporates herein by reference Japanese Patent Application No. 9-201141 filed on Jul. 28, 1997.
1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus and method for an internal combustion engine provided with a fuel evaporative emission purge system for introducing fuel evaporative gas adsorbed by a canister to an intake path of the internal combustion engine.
2. Description of Related Art
Fuel evaporative gas introduced or purged from a canister to an intake path of an internal combustion engine (purge gas) contains fuel. Thus, during the introduction of purge gas, the volume of fuel injected by a fuel injecting valve needs to be corrected by reduction of the fuel volume in accordance with the volume of the introduced purge gas in order to adjust the volume of the fuel supplied to the internal combustion engine to a required value. As disclosed in Japanese Patent Laid-open No. Hei 8-109844, however, some of the fuel injected from the fuel injecting valve is stuck to the internal wall of an intake pipe during the introduction of purge gas. As a result, the air-fuel ratio of air-fuel mixture is likely to deviate from a stoichiometric air-fuel ratio, or a target air-fuel ratio, to the lean side. For this reason, in the above air-fuel ratio control apparatus, an air-fuel ratio feedback correction coefficient is corrected to shift the air-fuel ratio to the rich side in dependence on deviations of the air-fuel ratio feedback correction coefficient detected before and after the introduction of purge gas. As a result, the air-fuel ratio of air-fuel mixture gas supplied to the internal combustion engine during the introduction of purge gas is converged to the stoichiometric air-fuel ratio.
In general, a three-way catalyst used for purifying NOx, CO and HC contained in exhausted gas has a narrow purifying range (window) only around the stoichiometric air-fuel ratio with a value ranging from 14.6 to 14.7 as shown in FIG. 15. It should be noted that the window implies a range of air-fuel ratios in which the purifying efficiencies of NOx, CO and HC are all high. Thus, air-fuel ratio feedback control must be carried out toward the stoichiometric air-fuel ratio used as a target air-fuel ratio even during introduction of purge gas.
According to results of a recent study, however, it has been found that the air-fuel ratio of the air-fuel mixture is shifted to the lean side from the window of the three-way catalyst even if the air-fuel ratio feedback control is carried out during introduction of purge gas. This is considered to occur as follows. As shown in Table 1 below, gasoline used as a fuel contains a number of hydrocarbon components of different types, and the stoichiometric air-fuel ratio as well as the boiling point vary from type to type. A stoichiometric air-fuel ratio in the range 14.6 to 14.7 of the fuel as a whole is actually an average value of the stoichiometric air-fuel ratios of these components.
Since purge gas introduced into the internal combustion engine is fuel evaporative gas evaporated from gasoline in a fuel tank, a number of hydrocarbon components each with a low boiling point are contained in the purge gas. As shown in Table 1, the smaller the carbon number (Cn), the lower the boiling point of the hydrocarbon. Thus, the purge gas contains a number of hydrocarbon components each with a low carbon number such as methane, ethane, propane, butane and pentane with carbon numbers C1, C2, C3, C4 and C5 respectively as shown in FIG. 16. The stoichiometric air-fuel ratios of these hydrocarbon components are in the range 17.24 to 15.36 which is higher than the range 14.6 to 14.7 of the stoichiometric air-fuel ratio of the fuel as a whole. Thus, during the introduction of purge gas, the stoichiometric air-fuel ratio of the fuel as a whole supplied to the internal combustion engine becomes higher than the stoichiometric air-fuel ratio of ordinary fuel which is in the range 14.6 to 14.7.
For the above reason, during the introduction of purge gas, if air-fuel ratio feedback control is carried out by using the normal stoichiometric air-fuel ratio which is in the range 14.6 to 14.7 as a target air-fuel ratio, the air-fuel ratio of the air-fuel mixture during the introduction of purge gas is shifted from the window of the three-way catalyst to the lean side, decreasing the efficiency of purifying of NOx.
It is thus an object of the present invention to provide an air-fuel ratio control apparatus and method for an internal combustion engine that is capable of optimizing air-fuel ratio feedback control during introduction of purge gas and increasing the efficiency of purifying of gas exhausted during the introduction of purge gas.
According the present invention, target air-fuel ratio is corrected or changed to a value on a fuel-rich side during introduction of purge gas due to the fact that the air-fuel ratio of the air-fuel mixture during the introduction of purge gas (fuel evaporative gas) is shifted from a window of a three-way catalyst to a fuel-lean side. As a result, since the air-fuel mixture ratio is subjected to feedback control toward the corrected target rich side air-fuel ratio during the introduction of purge gas, the shift of the air-fuel ratio of the air-fuel mixture to the lean side caused by the introduction of purge gas can be canceled by the correction of the target air-fuel ratio to the value on the rich side. The air-fuel ratio of the air-fuel mixture during the introduction of purge gas can thus be controlled to a value within the range of the window of the three-way catalyst, making it possible to increase the efficiency of purifying of the gas exhausted during the introduction of purge gas.
Preferably, the amount of correction of the target air-fuel ratio to a value on the rich side can be set in accordance with the volume of introduced purge gas. As the ratio of purge gas to fuel supplied to the internal combustion engine, that is, the concentration of the purge gas, rises accompanying an increase in volume of the introduced purge gas, the shift of the stoichiometric air-fuel ratio of the supplied fuel as a whole to the lean side also increases. Thus, by setting the amount of correction of the target air-fuel ratio to a value on the rich side in accordance with the volume of the introduced purge gas, the setting of the target air-fuel ratio during the introduction of purge gas can be further optimized. It should be noted that one of parameters such as the weight of the purge gas, the concentration of the purge gas, the flow rate of the purge gas and the control duty of a purge control valve employed in a fuel evaporative emission purge system can be appropriately selected to represent the volume of the introduced purge gas.
In addition, taking the differences in stoichiometric air-fuel ratio among gas components shown in Table 1 given above into consideration, the amount of correction of the target air-fuel ratio to a value on the rich side can be set in accordance with the volume of introduced purge gas and components of the purge gas. Thus, the amount of correction of the target air-fuel ratio to a value on the rich side during introduction of purge gas can be set with a higher degree of accuracy.